1. Field of This Invention
This invention relates to a rechargeable galvanic battery and especially to processes for its production. This is a battery with liquid sodium as the negative and liquid sulfur as the positive, electro-chemically active material, as well as with a ceramic solid electrolyte capable of conducting sodium ions; the sodium, in the case of this battery, is completely absorbed in a finely pored metal mat (felt) which fills the entire anode space.
2. Prior Art
Sodium-sulfur batteries having ceramic solid electrolytes, which are capable of conducting sodium ions, have been known for years [see, for example: J. T. Kummer and N. Weber, Autom. Eng. Congr. Detroit, S.A.E., (1967), page 670, 179; and Fally, Lasne and Lazennec, J. Electrochem. Soc. (1973), p. 1292]. In the case of the hitherto described constructions, which have been examined in more detail, the sodium usually is in the inside of a small tube consisting of the solid electrolyte, namely, .beta.--Al.sub.2 0.sub.3, which is surrounded by melted sulfur or sodium polysulfide, absorbed in a graphite matrix. In such batteries, the reaction of sulfur or sodium polysulfide with sodium to yield a sodium sulfide having a lower sulfur content is used for the production of an electric current. (In the following text sodium polysulfide having a high sulfur content, is also referred to and included in the term "sulfur".) The operating temperature of such cells or batteries as a rule are about 300.degree. C.
Several small tubes of solid electrolyte, in such a case, can be disposed in a common housing, which is filled with graphite felt and sulfur -- while the small open tubes lead to a common sodium supply vessel. In this way, a whole group of individual galvanic cells can be combined electrically in parallel connection; the housing which is in contact with the sulfur also serves as collector of the current.
It is also known, inversely, to bring up the sulfur electrode from inside and the sodium from outside to the same solid electrolyte tubes. In the case of such a design, the aggressive (active) sulfur melt does not come into contact with the housing material; also, the metallically-conductive sodium content leads to an even distribution of current between the housing and the individual cells, which is not quite guaranteed in the case of the sulfur electrode.
In the case of the above described arrangement of sulfur-filled small solid electrode tubes in a container with sodium it is necessary to absorb the sodium supply in a matrix of, for example, iron felt (mat) (see German appln. Nos. 1,771,029 and 2,401,726). This measure fulfills a double purpose; for one thing, any escape of liquid sodium in the case of damage to the battery housing, and any danger caused thereby, is prevented; secondly, the capillary system leads to the fact that the entire electrolyte surface remains covered with sodium in any charging state of the cell or battery, so the electric current density will be almost equally high everywhere; (otherwise the solid electrolyte would be, on a point by point bases, loaded with too high a current density, which would cause changes leading destruction of the solid electrolyte after a short period of time).
A uniform current load of the solid electrolyte, however, will appear only whenever care is taken to achieve uniform contact between sodium and electrolyte. In order to achieve this it is known to adjust (set) a definite narrow gap between the metal felt and the electrolyte by special measures (see DT-OS No. 2,401,726 and DT-OS No. 2,400,202. Such so-called distribution mechanisms for the liquid sodium along one wall of the small electrolyte tube have, however, the disadvantage that they will fulfill the desired purpose only up to a certain discharge; in order to avoid damage, it is therefore necessary in the case of such a battery that a relatively high minimum charge, and thus a minimum quantity of sodium, be maintained in the anode space (although for economic reasons, one should strive for a battery with as low as possible a discharge capacity, without danger of damage to the solid electrolyte).
In addition, the production of such known batteries, respectively of a definite narrow gap on the wall of the solid electrolyte in the anode space causes considerable difficulties, because as a result of the customary processing, such as drilling, milling or stamping or even only as a result of too high pressure, the pore structure of the metal felt will be influenced disadvantageously, at least in the transition area toward the solid electrolyte. The pores in the metal felt are indeed either crushed or plugged up by such processing methods and thus the pore structure of the fine-pored metal felt, which is essential for the functioning, will be destroyed thereby. The discharge capacity, the specific energy, the useful life of the battery or other characteristics are considerably impaired in this way.